Chemical transmitters are small molecules or pepfides that are synthesized in neurons. There are eight classical and generally accepted low-molecular weight transmitter substances: acetylcholine, dopamine, norepinephrine, serotonin, histamine, glycine, glutamate and .gamma.-aminobutyric acid (GABA). The catecholamines, dopamine, norephinephrine and epinephrine (derived from the amino acid tyrosine), the indolamine, serotonin, (derived from the amino acid tryptophan) and histamine (an imidazole) together comprise the "biogenic amines".
In response to an appropriate stimulus, chemical transmitters are released into a synaptic cleft (gap) where they either bind to a postsynaptic neuron or effector organ; or are removed. There are three mechanisms by which nervous tissue disposes of soluble or unbound transmitter substances: 1) diffusion, 2) enzymatic degradation (e.g. monoamine oxidases degrade biogenic amines); and 3) reuptake by high affinity, chemical transmitter specific uptake mechanisms.
The chemical transmitter serotonin is known to mediate stimulation or inhibition of a variety of smooth muscles and nerves, thereby influencing secretion by exocrine and endocrine glands and functioning of the respiratory, cardiovascular and central nervous systems. Serotonergic neurons are known to be involved in control of sleep, appetite, nutrient selection, blood pressure, mood, endocrine secretion, aggressivity, and numerous other sensitivities to external stimuli.
Certain chemicals or drugs are known to affect serotonin level or activity, thereby producing a therapeutic effect. For example, it has been found that endogenous serotonin levels can be increased by administering its precursor tryptophan. In addition, the drugs fluoxetine (Prozac) and trazodone are thought to increase the availability of serotonin in the presynaptic cleft by blocking its reuptake by presynaptic neurons. These drugs have shown clinical utility for treating depression and stimulating weight loss. Two other "serotonin uptake blockers", fenfluramine and its principal metabolite norfenfluramine, have also shown clinical utility in treating depression and obesity. In addition to blocking the uptake of serotonin by postsynaptic neurons, norfenfluramine appears to enhance serotonin release from pre-synaptic vessicles.
Although serotonin uptake blockers are considered the most effective class of antidepressants (when compared to monoamine oxidase inhibitors such as phenelzine, and tricyclic compounds, such as imipramine and amitriptyline), currently available compounds have associated side effects. For example fluoxetine (Prozac.RTM.) has been linked to aggressive behavior.
New compounds that increase the availability of serotonin at serotonergic neuronal gap junctions in vivo would be useful for treating a variety of diseases and conditions associated with inadequate serotonin-mediated nerve transmission.